This invention relates to liquid coating compositions which are capable of being highly pigmented and which contain little or no solvent.
Aminoplasts and/or polyester based coating compositions which are free of or containing low amounts of solvent are known.
German Published Application DAS No. 1,101,667 describes solvent-free aminoplast-based enamels obtained by combining aminoplast solutions with those plasticizers which were conventional at the time the application was filed, and then distilling off the solvent. Plasticizers indicated as especially suitable therein are products containing hydroxyl groups, e.g., castor oil. However, plasticizers which do not contain hydroxy groups are likewise utilized, such as petroleum distillates or conventional phthalate plasticizers. The coatings produced from such coating compositions by baking have poor solvent resistance and a low stability with respect to chemicals. They adhere poorly to metals and have unsatisfactory mechanical properties.
It is also known from German Pat. No. 1,231,833 that coating compositions which are cured at room temperature can be produced from the above-described coating compositions of DAS No. 1,101,667 by the addition of curing agents. An improvement in the coating properties is not attained thereby.
Aminoplasts are known from German Unexamined Laid-Open Application DOS No. 1,595,857 which can be used per se in a solvent-free form, but the coatings produced therefrom are hard, very brittle and inelastic. Combinations of these special aminoplasts with conventional varnish resins are said to be capable of processing in solvent-free form. However, such mixtures have a very high viscosity and therefore can be applied only with great difficulties. Moreover, they lead to coatings which are no better than those of conventional raw materials.
DOS No. 1,644,848 describes a solvent-free coating agent consisting of a reaction product of (a) an etherified aminotriazine/formaldehyde condensation product with a cyclic carboxylic acid, and (b) a fatty-acid-free polyester containing hydroxyl group and having an OH-constant of more than 4%. The coatings produced in accordance with the example set forth in this application (Comparative Example A) are transparent and show good mechanical properties. If the binders are used in pigmented coatings, the film properties of the coatings are markedly impaired.
The binders described in U.S. Pat. No. 3,852,375 of Biethan et al. issued Dec. 3, 1974 made up of aminoplasts and ester mixtures produce high quality, transparent coatings after a solvent-free or low-solvent application. The good mechanical properties are decreased in case of high degrees of pigmentation and then no longer meet absolutely the practical requirements. The coatings also tend to run off vertical surfaces during the baking step. This patent does not disclose the fact that if hexahydroterephthalic acid is utilized in predominant quantities in forming the polyester component, excellent coating compositions are formed which can be pigmented to a high degree.
U.S. Pat. No. 3,553,284 of Riemhofer et al. issued Jan. 1, 1971 describes solvent-containing coating compositions formed from polyesters which contain hydroxyl and carboxyl groups, and polymers and/or oligomers which contain N-methylol groups and/or N-methylol ether groups. The diol component of these polyesters consists of 80-100 mol-% of either ethylene glycol or of 1,2-propanediol and 0-20 mol-% of other diols. Hexahydroterephthalic acid is primarily employed as the acid component plus up to 20 mol-% of other dicarboxylic acids. Due to the tendency toward crystallization exhibited by the polyethylene hexahydroterephthalates, as well as due to the unsatisfactory mechanical properties of the coatings of low-molecular polypropylene hexahydroterephthalates at a high degree of pigmentation, the coating compositions of the aforementioned reference are unsuitable for the production of high-quality, solvent-free and/or low-solvent coating compositions. The reference does not disclose that the coating compositions described therein can also be processed in a solvent-free or substantially solvent-free condition.
German Application No. P 23 43 436 described liquid coating compositions formed up of binders and small proportions of organic solvents, as well as optionally also on a solvent-free basis, which contain as the binders mixtures of aminoplasts and polyesters. This application does not disclose that hexahydroterephthalic acid must be used in a predominant proportion in the acid component.
For examples of prior art polyesters and aminoplasts useful for forming coating compositions, see, e.g., U.S. Pat. Nos. 3,668,275, 3,668,276, 3,668,277, 3,678,128, 3,691,258, 3,770,668, and the references cited therein, whose disclosures are incorporated by reference.
It is an object of the present invention to provide solvent-free or low-solvent coating compositions which, in addition to having good general characteristics, such as excellent hardness in combination with good elasticity, are highly pigmentable and have coating and curing characteristics such that vertical surface areas can be coated therewith without the coating composition being nonuniformly distributed over the entire surface area due to gravity. Other objects will be apparent to those skilled in the art.